docs: def _get_unique_categories(self, df):
          """Get all categories for each categorical columns from training data."""
  
          categories = []
          for col in self._categorical_columns:
              categocial = pd.Categorical(df[col])
              col_categories = list(map(str, categocial.categories))
              col_categories.append('_UNKNOWN')
              categories.append(col_categories)
          return categories